Hollow, plastic containers are typically manufactured using a stretch blow molding machine. A plastic preform or parison is created utilizing an extruder, in an injection-molding operation. The parisons are conveyed to the stretch blow molding machine and inspected by automated visual scanners to detect irregularities. Any parisons failing to meet preprogrammed criterion are rejected before being loaded into the stretch blow molding machine. In general, the first step of the stretch blower molding process is to heat the parison in an oven upstream from the stretch blow molding machine rotary table. The parisons are conveyed through the oven and heated while moving continuously through the oven. Typically, the heated parison is transferred to the rotary stretch blow molding machine load station. The load station includes rotating arms that grasp the parison and place the parison in the two-piece mold cavity. The load station is synchronized With the stretch blow molding machine wheel and the mold closes around the parison after the parison is loaded.
As the wheel continues to rotate, the stretch rod is driven down through the neck of the parison. When the stretch rod is moving down, low-pressure air (typically 150 psi) is introduced into the parison via the hollow stretch rod. Some time after the stretch rod reaches its maximum extension or stroke, high pressure process air (typically 600 psi) is applied to the inside of the parison to force the heated plastic against the mold surrounding the parison. The mold is typically cooled by chilled process water circulated in the jacketed mold. The high-pressure process air is applied to the now formed bottle for a portion of the wheel's revolution to allow the bottle to cool.
After the cooling interval, the center rod retracts and the air seal is broken allowing the process air to exhaust. The stretch rod retracts to the fully retracted position before the mold opens to expose the bottle for extraction. The extraction or unload station is synchronized to the stretch blow molding machine wheel and transfers the bottle from the wheel to the exit conveyor. The exit conveyor transports the bottle to another inspection station and then on to a palletizer.
Stretch blow molding machines are available in many configurations ranging from two stations to 40 or more stations. Previously, there has been no way to monitor specific process parameters at the individual stations used to produce a bottle. Previous systems were only able to monitor process parameters as they related to all stations on a wheel. Process air pressure and water values could only be determined at the manifold level. There was no method to monitor the displacement of the stretch rod. Station molds, stretch rods, valves, seals and other unique components wear at different rates, are subject to different alignment errors during normal operation, mold changes and routine maintenance activities. These differences affect the quality of bottles produced. Significant station to station differences are difficult or impossible to detect during continuous production. Quality problems are detected only during random quality control samples. If a bad bottle is found during this quality check, it is impossible to determine how many out of specification bottles have been produced.
A stretch blow molding machine may produce as many as 30,000 or more bottles per hour. The wheel is totally enclosed by safety walls and doors. The high angular velocities make it difficult to visually observe station actions without the use of a strobe light. Very small process changes negatively affect the quality of the bottle. Most of these process changes are not detectable external to the machine. Even if slip rings were employed to provide power to the wheel, communicating across these rings severely limits the data through put due to the high noise margin induced by slip ring brushes.
A need has thus arisen for a system for monitoring workstation parameters of a stretch blow molding machine.